The human heart is divided into four chambers, two upper chambers called atria and two lower chambers called ventricles. The heart's function is to pump blood through the body's circulatory system. A healthy heart at rest typically beats between 60 and 100 times per minute and will pump over 1,800 gallons of blood per day. Each normal heart beat is the result of electrical signals generated at a precise area in the right atrium, called the sino-atrial node, the heart's natural pacemaker. These electrical signals cause a physical contraction of the atria, which pump blood into the ventricles. The electrical impulses then continue to the ventricles, causing them to contract and distribute blood throughout the body.
Arrhythmias, abnormal rhythms of the heart muscle, arise from numerous causes, including tissue damage due to previous heart attacks, congenital defects and certain diseases. Arrhythmias can originate in either the atria where they are generally not life-threatening, or the ventricles, where they can significantly interfere with the pumping of oxygenated blood and can therefore be life-threatening. During an arrhythmia, the heart beats either too slowly or too rapidly. An abnormally slow heart rate, generally defined as a heart rate below 50 beats per minute, is known as bradycardia. This condition is usually treated by implanting a bradycardia pacemaker, a device that monitors the heart and delivers electrical impulses when necessary to increase the heart rate. A more serious arrhythmia occurs when the ventricles beat at an abnormally rapid rate, a condition known as ventricular tachycardia. In ventricular tachycardia, abnormal electrical signals occur in the ventricles. When the ventricles beat at an abnormally rapid rate, they do not have sufficient time to fill with blood prior to each contraction and therefore less blood is pumped out of the heart. As a result, less oxygen is carried to the tissues and organs of the body. This lack of oxygen can cause dizziness, unconsciousness, cardiac arrest and, ultimately, death.
Episodes of ventricular tachycardia occur unpredictably and tend to become more serious over time. Ventricular tachycardia can progress to the most serious type of cardiac arrhythmia, ventricular fibrillation. In ventricular fibrillation, the heart's normal electrical impulses become disorganized and erratic. Unlike ventricular tachycardia, during which the heart continues to contract in an organized fashion, in ventricular fibrillation the heart quivers and ceases to pump blood. As a result, the individual's blood pressure falls to nearly zero. If ventricular fibrillation is not terminated quickly, the individual will experience a sudden cardiac death (SCD) episode during which the individual will become unconscious as a result of the heart's failure to pump oxygenated blood to the body's tissues and organs, and without prompt medical intervention, typically will die.
A well-known device for treating patients with arrhythmias is an implantable cardioverter/defibrillator (ICD) which is an electronic device that is implanted in the patient and is designed to monitor the patient's heartbeat and deliver electric pulses or shocks to the heart to terminate arrhythmias. A typical ICD system consists of a device for pulse generation, defibrillation leads and pacing/sensing leads. The pulse generator contains the battery and electronic circuitry that monitors the patient's heartbeat and delivers therapy upon detection of a ventricular tachyarrhythmia. The pacing/sensing leads are insulated wires that connect the pulse generator to the heart and allow the device to sense the patient's heartbeat. These leads also carry electrical pulses for pacing. The defibrillation leads carry electrical shocks to terminate ventricular tachycardia and ventricular fibrillation. The defibrillator is surgically implanted beneath the skin in the patient's abdomen and the defibrillation leads are typically either epicardial patch electrodes connected to the exterior of the heart or endocardial leads inserted transvenously into the chambers of the heart. An endocardial lead system may also include a subcutaneous patch electrode. An ICD system of this type is described in U.S. Pat. No. 5,014,701 to Pless et al., which is assigned to the assignee of the present application and which is incorporated herein by reference.
An important feature of an ICD is the arrhythmia detection system. One of the earliest techniques for detection is described in U.S. Pat. No. Re 27,757 to Mirowski in which a pressure transducer is positioned in the right ventricle of a patient's heart. When the sensed pressure falls below a preset threshold, the device determines the presence of an arrhythmia and a therapy is delivered. More recent ICD systems rely primarily on an evaluation of the sequence of cardiac event timing intervals from a sensed electrogram (ECG). Various algorithms are applied to the detected timing intervals to determine the presence of an arrhythmia. Ventricular fibrillation is typically detected based strictly on heart rate (or interbeat interval) while tachycardia is detected based on rate along with other parameters such as sudden onset, stability, sensed physiological activity (exercise) and ECG waveform morphology. Certain rate boundaries are programmed into the ICD for each patient and these boundaries set up specific detection zones. These systems are not entirely satisfactory because there is still difficulty in making certain determinations, such as for example distinguishing between ventricular fibrillation and atrial fibrillation. An inappropriate defibrillation shock is very painful to the patient and may actually induce fibrillation. It would therefore be desirable to have another detection system which could be used independently or in conjunction with prior art detection systems for detecting ventricular fibrillation.
Researchers have found that the electrical activity of the heart reflects the activity of a dynamical system. A dynamical system is a system which is time dependent and may be described with differential equations having at least three independent dynamical (time dependent) variables. The equations must contain a nonlinear term which couples several of the variables. This coupling is a manifestation of what can be considered as feedback. The theory used to describe such systems is known as chaos theory and systems which exhibit this type of behavior are called dynamical or chaotic systems. Dynamical systems such as the heart can exhibit both periodic and chaotic behavior depending on certain system parameters. These parameters appear as constants in the differential equations describing the system. The chaotic behavior exhibited by the heart is not immediately obvious when looking at an ECG. One way which investigators have used for observing the chaotic behavior of the heart has been to plot the interbeat spacing at a time n against the interbeat spacing at time n+1. Such a plot is known as a Poincare map or return map. Using chaos theory as a tool to characterize tachyarrhythmias and as a basis for arrhythmia control would thus be beneficial.
A system which uses chaos theory to diagnose vulnerability of patients to cardiac arrhythmias is disclosed in U.S. Pat. No. 5,201,321 to Fulton. An analysis is applied to R--R interval measurements to generate a specific indication of vulnerability to lethal myocardial infarction. This technique is not, however, used for the characterization of fibrillation as is needed in an ICD.
It is an object of the invention to provide an improved system and method for detecting ventricular fibrillation.
It is a further object of the present invention to utilize the dynamical nature of the heart as exhibited in ECG signals to detect fibrillation.